1. Field of the Invention
The present invention relates to an apparatus of sensing a position for a recording and reproducing device.
2. Description of the Related Art
An example of a magnetic recording and reproducing apparatus of recording and reproducing information stored on a magnetic tape using a rotary magnetic head is a video tape recorder (VTR). As is widely known, the picture quality and the functions of a VTR have been rapidly enhanced until now since the VTR was developed. The present state is that a development of a digital VTR recording digital signals continues.
The VTR recording and reproducing information on and from the magnetic tape using the rotary magnetic head has been usually used in recording continuous images such as contents of a television broadcasting program and video signals from a television camera. There is now an attempt to employ the VTR as a component of a system such as a storage of a digital information processing system.
When the digital information processing system employs the VTR as its component such as an auxiliary external storage of a computer and an apparatus of sequentially recording images of computer graphics at the end of a computation and then successively reproducing the images to produce an animation, the VTR must reliably and stably record information on a unit of one to a few recording tracks on the magnetic tape, since the digital information processing system often stores the information on the unit of one to a few recording tracks.
When the VTR with the rotary magnetic head records information on a particular recording track on the magnetic tape or rewrites recorded information on a particular recording track on the magnetic tape, the VTR: (1) previously prerolls the magnetic tape before a record start position, passes the magnetic tape from before the record start position and past the record start position at a predetermined tape speed when recording, and starts to record information on the predetermined number of recording tracks on the magnetic tape from the record start position or: (2) produces a control pulse, controls a rotational position of the drive shaft of a capstan motor to stop the magnetic tape at a recording position and then cause the rotary magnetic head to record information on the stopped magnetic tape.
The first prior-art method is now employed in an assembly recording mode and an insert recording mode of an editor function of the VTR. The first method is unsuitable to the case in which the VTR frequently records since it requires repeated cycles of magnetic tape rewind and feed and heavily loads a tape travelling mechanism and the magnetic tape. In addition, it is difficult to accurately select a recording track on the magnetic tape since a relative movement of the rotary magnetic head to the travelling magnetic tape determines the recording track on which the rotary magnetic head records information. In addition, it is impossible to reduce time intervals in recording since the prerolling is time-consuming.
The second prior-art method lightly loads the tape travelling mechanism and the magnetic tape, since the second method eliminates the need for the rewinding of the magnetic tape which is required by the first method and the rotary magnetic head records on the stopped magnetic tape. It also enables the recording track on the magnetic tape to be easily accurately selected. A track pattern produced on the magnetic tape in recording differs from a track pattern that produced in response to a locus of the rotary magnetic head on the magnetic tape travelling at a predetermined tape speed since the rotary magnetic head records on the stopped magnetic tape. When the rotary magnetic head plays back at the predetermined tape speed the magnetic tape which has recorded information by the second method, the rotary magnetic head cannot accurately follow the recording track. In addition, it is difficult to employ a magnetic tape in such a manner that the same magnetic tape contains both a track which recorded information by the second method and a track which recorded information while travelling at a predetermined tape speed.
In order to overcome problems in the above-described first and second prior-art method, a VTR has been proposed which comprises an actuator controlling the rotation of the rotary magnetic head in an intermittent recording in response to positional signals of the rotary magnetic head produced in response to absolute positions of the rotary magnetic head on a reference edge of the magnetic tape so that a locus of the rotary magnetic head produces the same recording track on the magnetic tape in a position of stopping every time the magnetic tape has intermittently travelled predetermined distances in the direction of travelling of the magnetic tape as a recording track on the magnetic tape in a position of travelling at a predetermined tape speed which is produced in response to a locus of the rotary magnetic head on the travelling magnetic tape.
This VTR requires a stable accurate detection of positions of the rotary magnetic head. In this connection, Japanese unexamined patent application publication SHO.52-117105, for example, discloses a prior-art apparatus of which a strain gauge sticking to an electromechanical device or transducer made of an electrostrictive material detects a displacement of an actuator in order to detect a movement of the rotary magnetic head. However, this apparatus cannot detect the positions of the rotary magnetic head although it differentiates the detected displacement of the actuator to produce a speed signal of the actuator and feeds the speed signal back to a drive circuit for the actuator, thereby suppressing a free oscillation of the actuator. This apparatus, which employs an amount of warpage in the electrostrictive material of the electromechanical device for detecting the displacement of the actuator, cannot accurately detect the positions of the rotary magnetic head since the amount of warpage has a hysteresis and the state of the warpage depends on a temperature.
Therefore, the assignee in Japan of this invention previously proposed a system sensing positions of a recording and reproducing device and free from the problems in the apparatus of the above patent application publication. This system comprises two positional sensors of the same input-output characteristic for the recording and reproducing device, out puts of the positional sensors complementing each other independently of the positions of the recording and reproducing device, and means for producing a differential signal from the outputs of the positional sensors.
This system increases the accuracy of a positioning of the recording and reproducing device, fully nulls a drift in the normal or home position of the recording and reproducing device, produces positional signals of a good S/N by means of an addition of signal components and a subtraction of random noise components, and nulls external hoises such as an external disturbance light by means of the subtraction. This system also approximately compensates changes in the outputs of the positional sensors due to a change in environment temperature by means of adding a simple temperature compensating circuit to the system when environment temperature narrowly changes. On the other hand, when environment temperature widely changes, the temperature compensating circuit cannot fully compensate changes in the outputs of the positional sensors due to a change in the luminous energy of light from a light emitting diode and due to a change in the photoelectric current of a photodiode. Therefore, measurements for improving a drawback in this system have been demanded since the system has a large error.